Slide 1

Slide 2 The Doppler Effect Slide 3 What is the Doppler effect? Youve heard the Doppler effect before. Maybe you were listening to the siren of an ambulance and heard the frequency change as the vehicle passed you. This change in frequency is the Doppler effect, but why does it happen? Lets explore this phenomenon with some animations. Please click on the link below and WATCH the simulation. Dont play with it yet!! Continue through the power point. http://highered.mcgraw- hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites /dl/free/0072482621/78778/Doppler_Nav.swf::Doppler%2 0Shift%20Interactive Slide 4 Observer and Source Are Stationary The Doppler effect is related to the frequency of the sound heard by an observer compared to the frequency of the sound the source actually produces. f s = frequency produced by the source f o = frequency heard by the observer How does f s compare to f o when the source and observer are stationary? Look at the simulation to answer this question. DONT PUSH ANY BUTTONS YET! Compare the rate at which the crests (green) and troughs (yellow) are being created to the rate at which the observer is hearing them. Slide 5 Observer and Source Are Stationary Without movement there is no Doppler Effect f o = f s Slide 6 Observer Stationary, Source Moving How does f o compare to f s when the source is moving towards the observer? How does f o compare to f s when the source is moving away from the observer? Use the simulation to answer these two questions. Click on the Source approaches button. In your notes, write down your observations and try to justify why the frequency changes in the way it does. Slide 7 Observer Stationary, Source Moving How does f o compare to f s when the source is moving towards the observer? f o > f s The speed at which the wave crests move remains constant. The frequency is larger because the wavelength decreases since the source moves in the direction the wave is moving, not allowing as much space between crests. Note: The source is moving to the left. If the source is moving towards the observer, the wavelength is small, the waves look bunched up. Slide 8 Observer Stationary, Source Moving How does f o compare to f s when the source is moving away from the observer? f o < f s The speed at which the wave crests move remains constant. The frequency is smaller because the wavelength increases since the source moves in the opposite direction the wave is moving, resulting in lots of space between crests. Note: The source is moving to the left. If the source is away from the observer, the wavelength is big, the waves look like they are spread apart. Slide 9 Observer Moving, Source Stationary How does f o compare to f s when the observer is moving towards the source? How does f o compare to f s when the observer is moving away from the source? Use the simulation to answer these two questions. Click on the Observer approaches button. In your notes, write down your observations and try to justify why the frequency changes in the way it does. Slide 10 Observer Moving, Source Stationary How does f o compare to f s when the observer is moving towards the source? f o > f s The frequency is larger because the time between hitting crests decreases since after hitting one crest the observer moves towards the source, hitting another crest sooner than they would if they were stationary. Slide 11 Observer Moving, Source Stationary How does f o compare to f s when the observer is moving away from the source? f o < f s The frequency is smaller because the time between hitting crests increases. After one crest overtakes them, the observer moves away from the source and the next crest takes longer to overtake them than if they were stationary. Slide 12 What does the f o depend on? We know that the frequency increases if the source is moving towards the observer or the observer is moving towards the source. What does the value of f o depend on? You can now play with the simulation for a few minutes. Note that you can reset the animation and then make either the source or the observer move by grabbing onto it and moving it in a particular direction. You will see an arrow pop up the bigger the arrow, the faster it will move. Slide 13 What does the f o depend on? You should have noticed that the larger the speed of the source or observer, the larger the resulting frequency change. What would happen if the observer was moving away from the source, but the source was moving toward the observer? Try to come up with a general trend for whether an observer hears a higher or lower frequency, no matter who is moving and in which direction. Slide 14 What does the f o depend on? No matter which direction the source and observer are moving: If the observer and the source are getting closer to one another, f o > f s If the observer and the source are getting further away from one another, f o < f s Slide 15 The Doppler Effect Equation!! Slide 16 Slide 17 Example 1 A police car is at rest with its siren on. Imagine that the siren plays a single tone of frequency 550 Hz. You are in your truck, moving toward the car at a speed of 18 m/s. (A) Is the frequency you hear greater than, equal to or less than 550 Hz? Greater than!! The source and object are moving toward each other. Slide 18 Example 1 A police car is at rest with its siren on. Imagine that the siren plays a single tone of frequency 550 Hz. You are in your truck, moving toward the car at a speed of 18 m/s. (B)Find the frequency you hear. f s = 550 Hz v o = 18 m/s v s = 0 v = 340 m/s f o = ? v s = 0 so we are left with this. We know that the frequency increases, so we must be using the + sign. Slide 19 The Doppler Effect Equation!! Moving toward Moving away